The waterways of the United States have served many practical needs, such as large-scale transportation of both commodities and manufactured goods, recreational activities, water front living environments, and oil, gas and mineral supplies. The utilization of these waterway resources has involved the construction and maintenance of many structures including harbors, locks, canals, marinas, breakwaters, shore protection and water intakes so that, among other things, ships and marine equipment may be operated in all kinds of weather conditions. It is estimated that up to 95 percent of the commerce in the United States depends on ocean transportation. For example, in 1994, 4,537 ships were called at port in New York Harbor, each carrying the equivalent of 1,000 railcars or 2,000 trucks which represents 12 percent of the total ocean commerce in the United States.
It has been found, however, that dredging is often required to maintain water depth in waterways such as harbors and channels. For example, New York Harbor is a natural shallow estuary fed by the Hudson, Passaic and Hackensack Rivers. It receives between about three and five million cubic yards of sediment drift each year, creating shoals that threaten to block its 240 miles of navigational channels. The natural depth of New York Harbor is only about 18 feet, while most cargo ships require at least a 40-foot draft.
The build up of sediment further decreases the depth of New York Harbor and, thus, large fully loaded container ships may no longer be accommodated. As a result, many vessels must unload part of their cargo on barges in deeper parts of the Harbor or at other ports. After lightening, the container ships float high enough in the water to be docked. Similarly, oil tankers must perform a procedure called "lightering" in which some of the crude oil is pumped into smaller barges out in the ocean. These types of procedures greatly impact the cost of transportation as well as increasing the risk of environmental contamination from oil spills. As such, the livelihood of harbors, such as New York Harbor, depends on the continued clearing of silt therefrom.
It has been found, however, that dredged sediments are often contaminated with pollutants. Specifically, dioxins, PCBs, heavy metals including lead, mercury and cadmium, pesticides, as well as other contaminants from waste water discharges and run off attach themselves to the silt that washes into harbors. In the past, silt and muck dredged up from the port of New York and New Jersey has been dumped in the ocean in a 2.2 square mile area known as the Mud Dump.
Since 1977, the Environmental Protection Agency ("EPA") has been testing sediment that is dumped into the ocean using a pass/fail test. The EPA's regional office in New York, however, enforces a more stringent guideline for New York Harbor. Dredged materials are classified into three categories: Category I means the sediment is clean enough to dump in the ocean; Category II mud has trace levels of chemicals which could formerly be dumped in the ocean and "capped" with clean sand but presently has been barred from disposal in the Mud Dump; and Category III is so contaminated that federal law prohibits ocean disposal. It has been estimated by the Army Corp of Engineers that two-thirds of the sediment in New York Harbor falls within Category III. An additional 20 percent of the sediment falls within Category II.
A variety of prior methods have been used to clear silt from navigational channels that do not require dumping the dredged materials into the ocean. For example, efforts have been made to level or reprofile the accumulated silt rather than removing it. At best, this approach yields a short term solution given the volume of sediment drift into harbors and channels. Other alternatives have been suggested to dispose of dredged materials. For example, digging an underwater pit to bury dredged materials that have been packaged in large containers or building a contaminant island to store the dredged material. Digging an underwater pit, however, could disturb heavy metals such as mercury, lead and cadmium below the sea floor and would require several years to prepare. Similarly, constructing a contaminant island may cost between $500 million and $1 billion and would take more than five years to prepare.
Alternatively, dredged materials have been transferred to landfills or other on land sites for disposal. This approach, however, may cost more than 20 times that of dumping in the ocean and requires the transfer and transportation of contaminated materials which creates a separate and further environmental risk. Additionally, prior methods for re-use of the dredged materials have not been successful. For example, prior attempts to use dredged material for resurfacing roads have yielded an asphalt with a limited life due to the low durability and friability of the end product.
Therefore, a need has arisen for a method and associated apparatus for the treatment of dredged materials that is cost effective on a large scale, that is environmentally sound and that creates a mixture that is suitable for a beneficial re-use as a structural fill.